Zhelin bay is characterized by intensive aquaculture along the coast of Southeast China, where approximately half of the water area are occupied by either cultured oysters or caged-fish farms. As one of the most important bays for large-scale mariculture in Guangdong Province, aquaculture production in Zhelin Bay has made significant contributions to the local economy. However, development of intensified caged-fish farms during the last decade has accelerated eutrophication process of the bay, and massive fish kills due to harmful algal blooms such as Phaeocystis pouchetii were reported in 1997 and 1999. Diatom blooms such as Skeletonema costatum and Thalassiosira diporocyclus have also been reported in recent years. Since May 2000, our research group has initiated an ecological investigation around Zhelin Bay, where samples were collected monthly or quarterly for analysis of phytoplankton, zooplankton, microorganism, water temperature, salinity, nutrients and other environmental parameters. In the present study, we described the seasonal dynamics of Rhizosolenia community in 9 sampling stations from May 2004 to February 2005.
Samples were collected at high tide (±1.5 h) quarterly during the investigation. For quantitative phytoplankton analysis, one liter of water samples were collected with 5-L HQM-1 sampling bottles 0.5 m under the surface and 0.5 m above the bottom at each station. Immediately after sampling, the samples were fixed with Lugol′s iodine solution with a final concentration of 15‰ and then transferred to graduated glass cylinders and concentrated to 30-100 ml gradually through sedimentation for a period of 3 days. Species identification and density counting was performed with inverted microscope (Zeiss Axiovert25, magnification 400×) using a 1-ml Sedgewick-Rafte counting frame, and a minimum of 400 cells were counted for each sample. Water temperature, salinity, turbidity, pH value, dissolved oxygen (DO) and water depth were determined in situ with a portable water quality meter (YSI6600-02, USA). Additional water samples (250-ml) were collected (filtered in situ) for nutrient and chlorophyll a analysis, and they were measured with an injection water quality analyzer (SKALAR, Netherlands) and a spectrophotometer (UV-2501PC, Japan), respectively.
A total of 72 phytoplankton samples were collected in the present study, and among which Rhizosolenia was found to be present in 64 samples, with total 16 Rhizosolenia species; R. delicatula, R. fragilissima, R. setigera, R. styliformis var. latissima were the species appeared in Zhelin bay all year round. The community density ranged from 0 to 25.8×103 cells/dm3 and an overall average of 5.04×103 cells/dm3, contributing 4.1% to total phytoplankton density.
Spatially, Rhizosolenia species abundance ranged from 4 to 14, and the density ranged from 2.18-11.02×103 cells/dm3, both were characterized by a typical increasing pattern with the lowest located inner and highest located outer of the bay. Temporally, Rhizosolenia species abundance ranged from 6 to 15, and the density ranged from 0.09-9.51×103 cells/dm3, both were characterized by a typical annual pattern with the lowest in cold seasons and highest in warm seasons.
In total, Zhelin Bay had high Rhizosolenia species abundance and mid-level community density. Our analysis indicated that water temperature was the key factor affecting seasonally the species richness and community density of Rhizosolenia in Zhelin bay, and the light、interspecific competition and structure of zooplankton community were important factors controlling its spatial and temporal distributions.